As is known in the art, systems and methods for creating large size, highly homogeneous surface and volume diffraction gratings are sought in many applications. In most cases, such gratings are created by interfering two large-size well-characterized optical beams having plane wavefronts on a recording media. If large-sized high-homogeneous holograms are required, large interfering beams have to be applied. To provide approximately the same exposure dosage over a recording media, just the central part of Gaussian beams is usually used for recording. It necessitates getting even larger interfering beams. Such beams are created by expanding the output of well-characterized, single-transverse-mode lasers from 1-2 mm diameter to several centimeters or even to several tens of centimeters. However, there are a few drawbacks of the application of high expanded beams: (1) Large-sized and rather expensive optics is needed for a large beam expansion; (2) There is still a considerable difference in the exposure dosage of different parts of holograms even in the case of very high magnification of Gaussian beams; (3) A large expansion is hard to implement without truncating the edges of the expanded beams on the various unavoidable apertures in any practical holographic set up. The beam truncation causes diffraction, which manifests itself as additional patterns in the hologram. These parasitic patterns modulate the dominant grating of the hologram and reduce the overall performance of the final product; and, (4) Truncating the edges of the expanded beams decreases the total usable power of the interfering beams, which necessitates undesirable lengthy exposures.
More particularly, interference patterns to record surface and volume holograms are usually created with wavefront-splitting interferometers, phase-masks, or amplitude-splitting interferometers, see, for example, Fibre Bragg Grating: fundamentals and application in telecommunication and sensing. A. Othonis and K. Kalli, Artech House, Boston, London, pp. 150-162, 1999. In the first technique, two interfering beams are carved from different areas of the wavefront of a spatially coherent beam. Such splitting, however, results in diffraction at the boundary of the cut, causing the parasitic interference fringes that have been described above. Moreover, this method requires an additional beam expansion if large-sized gratings have to be recorded.
In a second technique, a phase mask is illuminated by a single laser beam, creating interfering beams on a closely positioned target. Therefore, this technique is inapplicable for large-sized or thick grating recording.
In the third method, which is the most universal, two interfering beams are created by splitting a parent beam in two on a partially reflecting beam splitter. The beams are then spatially shaped and combined on a target. This technique has been used for large-sized hologram recording. However, it requires considerable beam expansion for achieving uniform illumination across large areas; therefore, this technique has several drawbacks. More particularly, small-scale distortions in the spatial distribution of the interfering beams result in hologram degradations similar to those caused by diffraction on apertures. The basic sources of such distortions are diffraction on dust particles or on inhomogeneities of optics, and interference between the main beam and the beams re-reflected from the different surfaces of optical setup and recording media. The spatial filters and a clean-room environment may be used to reduce partially the influence of small-scale distortions. However, all the distortions appearing after the spatial filter still result in hologram degradations. One of these drawbacks, namely a parasitic diffraction, can be suppressed by a method described in U.S. Pat. No. 3,834,786 issued Sep. 10, 1974, inventor W. Carlsen to record holograms of transparent optical objects. The patent describes a system imaging an aperture (which was required in the signal channel) on the target plane. Such a method, however, only partially removes unwanted interference patterns because of residual diffraction in the reference channel.